dmb

Data Memory Barrier (Thumb)

DMB <option>

Memory barrier (Thumb).

Pseudocode Operation

if option == SY then
  DataMemoryBarrier(FullSystem)
else if option == ISH then
  DataMemoryBarrier(InnerShareable)
else if option == OSH then
  DataMemoryBarrier(OuterShareable)
else if option == NSH then
  DataMemoryBarrier(NonShareable)
else
  DataMemoryBarrier(FullSystem)

Example

DMB option

Encoding

Binary Layout

111100111011

31:20

1

19

1

18

1

17

1

16

10

15:14

0

13

0

12

1

11

1

10

1

9

1

8

0101

7:4

option

3:0

 

Format Thumb System

Opcode 0xF3BF8F50

Extension A32 (Base)

Operands

• option
  SY/ISH

Related

Other forms of dmb

• dmb Data Memory Barrier
• dmb Data Memory Barrier (A32)

Across architectures

Memory Barrier / Fence : how x86, ARM, RISC-V, and PowerISA each do this.

More in A32 (Base)

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0xF57FF050	DMB{<c>}{<q>} {<option>}		A32		111101010111 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0101 | option
0xF3BF8F50	DMB{<c>}{<q>} {<option>}		T32		111100111011 | 1 | 1 | 1 | 1 | 10 | 0 | 0 | 1 | 1 | 1 | 1 | 0101 | option

Description

Data Memory Barrier is a memory barrier that ensures the ordering of observations of memory accesses, see Data Memory Barrier (DMB).

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    MBReqDomain domain;
    MBReqTypes types;
    case option of
        when '0001'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_Reads;
        when '0010'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_Writes;
        when '0011'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_All;
        when '0101'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_Reads;
        when '0110'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_Writes;
        when '0111'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_All;
        when '1001'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_Reads;
        when '1010'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_Writes;
        when '1011'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_All;
        when '1101'  domain = MBReqDomain_FullSystem;      types = MBReqTypes_Reads;
        when '1110'  domain = MBReqDomain_FullSystem;      types = MBReqTypes_Writes;
        otherwise    domain = MBReqDomain_FullSystem;      types = MBReqTypes_All;

    if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then
        if HCR.BSU == '11' then
            domain = MBReqDomain_FullSystem;
        if HCR.BSU == '10' && domain != MBReqDomain_FullSystem then
            domain = MBReqDomain_OuterShareable;
        if HCR.BSU == '01' && domain == MBReqDomain_Nonshareable then
            domain = MBReqDomain_InnerShareable;

    DataMemoryBarrier(domain, types);